Thermal dispersion mass flow rate, material interface, and liquid level sensing transducer

Abstract

A thermal mass dispersion flow rate sensing transducer and transducer assembly or instrument for improved functional life of the transducer without degradation in sensing accuracy. Several aspects of the transducer components and structure reduce thermal leakage within the transducer so the sensor (RTD) output signal is accurately transmitted to the signal processor, resulting in precise ΔT determinations and consequent precise determinations of mass flow rate of the fluid flowing in the conduit. Additionally, the same components and structure also have long life without appreciable degradation, thereby delaying any basis for the need for recalibration of the instrument.

What is claimed is: 
     
       1. A thermal dispersion transducer in a sensing instrument, the sensing instrument having a reference transducer and an active transducer, the active transducer being heated in order to provide a temperature differential (ΔT) signal with respect to a reference temperature transducer which provides an output indicative of the temperature of the media to be sensed, the reference and active transducers being structurally identical, the thermal dispersion transducer comprising:
 at least one elongated tubular thermal well shell having a proximal end connected to the instrument and a distal end configured to extend into the media to be sensed, said thermal well shell having a level of thermal conductivity that is equivalent to stainless steel; 
 a seal cap sealing the distal end of said shell, said seal cap having a surface that is internal to said shell and an external surface which is exposed to the media; 
 a thermal sensor fused to said internal surface of said seal cap, said sensor having lead wires extending therefrom; 
 a flux-free fusing material on said thermal sensor by which said sensor is fused to said seal cap; 
 a thermally insulative support element within said shell spaced from said seal cap; 
 a first thermally insulative material filling the space between said seal cap and support element, said lead wires passing through and being supported by said first thermally insulative material; 
 first single strand wires having thermal and electrical conductivity equivalent to stainless steel extending through said support element from said insulative material to the proximal end of said shell, said sensor lead wires being connected to said first single strand wires; 
 at least one electrically insulative cement filling said shell from said support element to the proximal end of said shell to seal said proximal end against air and moisture; and 
 a nipple to which the proximal end of said thermal well is mounted, said nipple being configured to provide external electronic access to said thermal sensor and to prevent ambient temperature from being communicated to the interior of said shell. 
 
     
     
       2. The transducer of  claim 1 , wherein said at least one cement is thermally conductive. 
     
     
       3. The transducer of  claim 1 , and further comprising a fin projecting out from said external surface of said seal cap. 
     
     
       4. The transducer of  claim 1 , wherein said thermal well shell is made of stainless steel. 
     
     
       5. The transducer of  claim 1 , wherein said first thermally insulative material is magnesium oxide (MgO) powder. 
     
     
       6. The transducer of  claim 1 , wherein said thermally insulative support element is made of aluminum oxide (Al 2 O 3 ) cement. 
     
     
       7. The transducer of  claim 1 , wherein said at least one electrically insulative cement comprises aluminum nitride (AlN) cement. 
     
     
       8. The transducer of  claim 1 , wherein said at least one electrically insulative cement comprises magnesium oxide (MgO) cement. 
     
     
       9. The transducer of  claim 1 , wherein said at least one electrically insulative comprises two different cement segments. 
     
     
       10. The transducer of  claim 9 , wherein said two different cement segments comprise a magnesium oxide (MgO) segment and an aluminum nitride (AlN) segment. 
     
     
       11. The transducer of  claim 1 , wherein said nipple has second single strand stainless steel wires connected at its distal end to said wires from said shell and at its proximal end to a signal processor. 
     
     
       12. The transducer of  claim 11 , wherein said nipple has an interior volume through which said second wires pass, said interior volume being filled with magnesium oxide (MgO). 
     
     
       13. A transducer sensing instrument for determining fluid mass flow rate, liquid level, non-liquid flow rate, or interface location for non-miscible liquids, the instrument having a thermal dispersion reference transducer and a thermal dispersion active transducer, the active transducer being heated in order to provide a temperature differential (ΔT) signal with respect to a reference temperature transducer which provides an output indicative of the temperature of the media to be sensed, the reference and active transducers being structurally identical, the instrument comprising:
 a pair of thermal dispersion transducers, each said thermal dispersion transducer comprising:
 at least one elongated tubular thermal well shell having a proximal end connected to the instrument and a distal end configured to extend into the media to be sensed, said thermal well shell having a level of thermal conductivity that is equivalent to stainless steel; 
 a seal cap sealing the distal end of said shell, said seal cap having a surface that is internal to said shell and an external surface which is exposed to the media; 
 a thermal sensor fused to said internal surface of said seal cap, said sensor having lead wires extending therefrom; 
 a flux-free fusing material on said thermal sensor by which said sensor is fused to said seal cap; 
 a thermally insulative support element within said shell spaced from said seal cap; 
 a first thermally insulative material filling the space between said seal cap and support element, said lead wires passing through and being supported by said first thermally insulative material; 
 first single strand wires extending through said support element from said insulative material to the proximal end of said shell, said sensor lead wires being connected to said first wires; 
 at least one electrically insulative cement filling said shell from said support element to the proximal end of said shell to seal said proximal end against air and moisture; 
 a nipple to which the proximal end of said thermal well is mounted, said nipple being configured to provide external electronic access to said thermal sensor and to prevent ambient temperature from being communicated to the interior of said shell; and 
 a signal processor for determining the mass fluid flow rate, liquid level, non-miscible liquid level, or non-liquid flow rate based on the temperature differential outputs (ΔT) of said active and reference transducers, the outputs of said active and reference transducers being connected through said nipple to said signal processor.